The present invention relates to a method for the electrolytic production of injection moulding moulds made of nickel in accordance with the preamble of claim 1.
Such methods are employed, among other things, for the electrolytic production of items made of metal which are difficult to produce according to other methods. In the production of optical media such as Compact Discs for example, the production of the individual CDs that are obtainable on the market is performed by using the injection moulding method. As is well known, an injection moulding mould is required for this purpose, with a so-called xe2x80x9cmasterxe2x80x9d being produced in a first working cycle which actually represents the only true original. In further working cycles a so-called xe2x80x9cmotherxe2x80x9d is produced which on its part represents the precise counterpart of the xe2x80x9cmasterxe2x80x9d. It is used to produced so-called xe2x80x9cstampersxe2x80x9d which are then used as the actual injection moulding moulds.
In order to produce these injection moulding moulds a glass plate is produced at first which comprises the data embossed in form of recesses or elevations on one of its sides on the surface which are produced by the application of photosensitive resist, with said surface characterized by the recesses and elevations having a shape representing the precise counterpart to the injection moulding mould to be produced. Said glass plate is now used as the workpiece and is connected in the next working cycle (within the galvano-technical process) as a cathode and the metal from which the actual product is to be made, which in the present case is nickel, is connected as an anode. Both the metal as well as the workpiece are held in this process in an electrolytic solution which substantially consists of distilled water, boric acid and nickel sulphamate. After the application of a DC voltage the metal, which may be present in the form of so-called nickel pellets, dissolves in the solution and travels in the direction towards the cathode, which means towards the workpiece, on the surface of which it deposits. Following the severing of the metal coating from the glass plate, the xe2x80x9cmasterxe2x80x9d has thus been produced. With the help of said xe2x80x9cmasterxe2x80x9d, which is now used instead of the glass plate, the production of the xe2x80x9cmotherxe2x80x9d occurs in an analogous manner. The production of the xe2x80x9cstamperxe2x80x9d, which constitutes the replacement of the xe2x80x9cmasterxe2x80x9d by the xe2x80x9cmotherxe2x80x9d, also occurs analogously. A stamper is thus also identical with the master and is now used as an injection moulding mould for producing the CDs.
The depositing speed of the nickel on the workpiece (master, mother) depends on the applied voltage, i.e. on the current intensity or density. In order to accelerate the production process it would be necessary to keep the current density as high as possible. The mean current density is approx. 2.5 A/dm2 in conventional methods.
The current which is currently used in practical operation concerns constant direct current. Problems will occur, however, in increasing the current strength which will have a negative influence on the quality of the deposit, i.e. the injection moulding mould, to such an extent that further use of the mould in the further production process is not possible. Thus, an increase of the current and thus the current density will cause an uneven nickel deposition on the workpiece, with the formation of local voltage peaks on the surface of the workpiece, thus preventing the obtained mould from achieving the required shape and even density. In the production of CDs, however, the adherence to very narrow tolerances is particularly important, since the laser which scans the stored data requires a precisely set distance to the CD in order to operate correctly. If the CD now has a varying thickness due to an imprecise irregular injection moulding mould, the distance between the laser and the CD surface will vary and read errors can occur. Moreover, the uneven distribution of material in CDs will cause balance errors which cannot be neglected and constitute a big problem in the use of modern CD-drives which, as is well known, have very high speeds of rotation. The uneven nickel deposition on the workpiece further leads to internal tensions in the mould, thus finally making the entire mould oblique and thus useless for the further production process. Another big problem in increasing the DC strength is the occurrence of so-called xe2x80x9crough backxe2x80x9d unevenness. This is understood as being uneven locations which occur on the surface of the mould that does not bear any data. Local current peaks will lead to increased deposits of nickel at certain locations, thus leading to elevations in the surface there, which again contribute to the formation of voltage peaks. This loop leads to the consequence that nickel deposits occur in form of hills or tips on the reverse side of the mould. As a result of the high pressure with which the molten material is pressed onto the mould in the injection moulding machine, these uneven locations push through the mould and damage the workpiece to be produced, namely the CDs that can be purchased.
In order to avoid these problems it was tried to use a known method, the so-called pulse-plating method, for producing such moulds. The pulse-plating method is already used in practical operation for the galvanotechnical production of metal objects. A pulsating direct current of various pulse forms (rectangle, sine, triangle) is used. It can also occur that the direction of current is reversed briefly, as a result of which xe2x80x9cbulgedxe2x80x9d deposits of material which are accumulated by local voltage peaks can be removed again. In this way it is possible to effectively prevent the formation of the xe2x80x9crough backxe2x80x9d unevenness and the mould to be produced is provided with a very homogeneous metal structure. In order to obtain the same depositing rate of nickel at the cathode by using the pulse-plating method for the production of nickel moulds as in the pure DC method, it is necessary to increase the mean density of current and thus also the peak current density as compared with the pure DC method in order to at least obtain or increase the net material transport. The increase of the peak current density for the application of the pulse-plating method was only possible up until now in the case of very stable electrochemical solutions (such as Cr). In the case of nickel solutions it is not possible to exceed a density of current of 2.5 A/dm2 as the nickel solution will otherwise begin to degrade. Moreover, the anode solubility (the nickel solubility) is too low at high strengths of current and the pH-value drops below 3.8, as a result of which the solution begins to degrade and becomes useless as electrolyte.
From JP 10060680 A it is known to add halogen ions to the electrolytic solution. A relevant increase of the current density is not possible with this method.
It is the object of the present invention to accelerate the electrolytic production of nickel workpieces, and nickel moulds in particular, without destroying the electrolytic solution and without any reduction in quality of the nickel mould which would prevent its further use.
As a result of the special concentration of the additives, the electrolytic solution will also remain stable at high mean densities of current, the pH-value will remain constant and the net material transport during the pulse-plating method can be at least maintained in comparison with pure DC methods with constant strength or density of current, and can also be increased with a respective increase of the mean density of current. Furthermore, the solubility of the nickel pellets is increased, thus reducing the time required for the entire production process.
A pulsating direct current or alternating current chosen in such a way that a mean density of current of between 40 ampere/dm2 and 120 ampere/dm2, preferably 50 ampere/dm2, is formed substantially helps to reduce the duration of the coating process (by a factor of 7) as compared with conventional methods.
The claims 2 to 5 describe preferable embodiments of the method.